We've had reports of anomalies from other sources that we also cannot replicate. It was suggested that it may because they were using a 64bit machine rather than 32bit, hence the question about hardware. Is your CPU 64bit?

In the jQuery SVN under ui/current/datepicker/test there is a JsUnit (http://www.jsunit.net) test suite for the datepicker. I've added a test for the suspect dates to the week-of-year tests. Can you run this test suite and report the results?